The soybean, Glycine max (L.) Merril, is one of the major economic crops grown worldwide as a primary source of vegetable oil and protein (Sinclair and Backman, Compendium of Soybean Diseases, 3rd Ed. APS Press, St. Paul, Minn., p. 106 (1989)). The growing demand for low cholesterol and high fiber diets has also increased soybean's importance as a health food.
Soybean yields in the United States are negatively affected each year by diseases. High yields per hectare are critical to a farmer's profit margin, especially during periods of low prices for soybean. The financial loss caused by soybean diseases is important to rural economies and to the economies of allied industries in urban areas. The effects of these losses are eventually felt throughout the soybean market worldwide.
Asian Soybean Rust (herein referred to as ASR) has been reported in the Eastern and Western Hemispheres. In the Eastern Hemisphere, ASR has been reported in Australia, China, India, Japan, Taiwan and Thailand. In the Western Hemisphere, ASR has been observed in Brazil, Columbia, Costa Rica and Puerto Rico. ASR can be a devastating disease, causing yield losses of up to 70 to 80% as reported in some fields in Taiwan. Plants that arc heavily infected have fewer pods and smaller seeds that arc of poor quality (Frederick et al., Mycology 92: 217-227 (2002)). ASR was first observed in the United States in Hawaii in 1994. ASR was later introduced into the continental United States in the fall of 2004, presumably as a consequence of tropical storm activity. Model predictions indicated that ASR had been widely dispersed throughout the southeastern United States, and subsequent field and laboratory observations confirmed this distribution.
Two species of fungi, Phakopsora pachyrhizi Sydow and Phakopsora meibomiae (Arthur) Arthur, cause ASR. Unlike other rusts, P. pachyrhizi and P. meibomiae infect an unusually broad range of plant species. P. pachyrhizi is known to naturally infect 31 species in 17 genera of legumes and 60 species in 26 other genera have been infected under controlled conditions. P. meibomiae naturally infects 42 species in 19 genera of legumes, and 18 additional species in 12 other genera have been artificially infected. Twenty-four plant species in 19 genera are hosts for both species (Frederick et al., Mycology 92: 217-227 (2002)).
Evaluating plants that could potentially contain QTL conferring resistance to ASR can be time consuming and require large amounts of biologically contained space. Culturing P. pachyrhizi requires the use of an approved biological containment hood. In addition, greenhouses and growth chambers used to grow plants for ASR resistance testing have to be constructed in a manner that prevents the accidental release of the organism, especially in locations in which the organism has still not yet been observed. Different cultures of P. pachyrhizi may possess different virulence factors. Over time, new strains of P. pachyrhizi may be introduced into the United States. The two principal hosts are yam bean (Pachyrhizus erosus (L.) Urban) and cowpea (Vigna unguiculata (L.) Walp.), both found in Florida. One widespread naturalized host for P. pachyrhizi and P. meibomiae is kudzu (Pueraria montana (Lour.) Merr. var. lobata (Willd.) Maesen & S. M. Almeida ex Sanjappa & Predeep). Because kudzu is a common weed in the southeastern United States, it might serve as a continual source of inoculum. Both P. pachyrhizi and P. ineiboiniae are autoecious (no alternate hosts) and microcyclic (with uredinial and tclial spore stages), with the obligate pathogens surviving and reproducing only on live hosts. Additional hosts can serve as over-wintering reservoirs for the pathogen, as well as build-up of inoculum. The pathogen is well adapted for long-distance dispersal, because the spores can be readily carried by the wind, making it an ideal means for introduction to new, rust-free regions. The primary means of dissemination are spores, which can be carried by wind or splashed rain.
Because different cultures of P. pachyrhizi may possess different virulence factors to known and suspected genes for resistance, it follows that different ASR resistance loci in the soybean genome may be expected to differ with respect to which strain(s) of P. pachyrhizi and or P. meibomiae they confer resistance against. Therefore, any breeding program designed to breed resistance into soybean against ASR will likely need to involve multifactorial resistance derived from different resistance loci in the soybean genome, in order to confer robust resistance against ASR despite changes in the P. pachyrhizi population. Also, breeding for soybean crops used in other geographic locations will require selecting resistance to the specific strains that affect those regions, in addition to providing those agronomic characteristics that are preferred by these farmers in that region. There is therefore strong motivation to identify novel ASR resistance loci in soybean, and to introgress desirable alleles into elite soybean germplasm. Methodology has been developed to evaluate plants that could potentially contain QTL conferring resistance to ASR (U.S. patent application Ser. No. 20080166699).